The present invention relates generally to incoherent beam combining and, more particularly, to a method and apparatus for generating a color display.
There is an increasing demand for large-screen information displays, both as data displays and entertainment sources. While rapidly developing plasma displays can fill some niches, front and rear projection systems are still the primary means used for those applications requiring a large screen. Projection-based display systems can be realized in several different ways and can utilize CRTs, liquid-crystal image plates or multipixel arrays.
One method of achieving a very bright, high resolution display is to scan a modulated laser beam across the display screen. The technology for scanners and modulators has developed rapidly in the last several years with micro-optical-mechanical systems (MOEMS) and micro-electro-mechanical systems (MEMS) demonstrating very high performance. The development of suitable laser sources, however, has lagged behind these developments in scanner/modulator technology, remaining both overly complicated and expensive. At least in part, the difficulties of utilizing lasers in display devices has been due to the inadequate power, beam brightness, color and durability of typical wavelength-converted solid-state lasers.
To date, diode lasers have been the simplest and lowest cost source for laser-based display systems. Unfortunately, green and blue diode lasers are not widely available nor do they produce sufficient power. For example, diodes emitting in the red wavelength band are available but produce only a few tens of milliwatts. Even worse, the green and blue diode sources under development have only been demonstrated with power levels of a few milliwatts. On the other hand, display applications typically require between 500 milliwatts and 5 watts of output power at all three primary colors.
Accordingly, what is needed in the art is a display system capable of high brightness and resolution using high power, inexpensive, durable laser sources capable of operating at the desired red, green and blue wavelengths. The present invention provides such a system.
The present invention provides a method and apparatus for generating a display. In the disclosed system, each color required by the display is generated by incoherently combining the beams of tens or hundreds of individual lasers, thus providing sufficiently high output for large displays. Additionally, as the lifetime of a diode laser is an inverse function of its output power, utilizing a large number of lasers for each color source increases the operating lifetime of the system since the individual lasers operate at relatively low powers.
Each incoherently beam combined (IBC) system used in the present invention is comprised of a plurality of gain elements/emitters. The individual IBC cavities are formed by the combination of a high reflectance (HR) coating applied to the back facets of the gain elements/emitters and an external output coupler. Within each IBC cavity is a wavelength dispersive element, such as a diffraction grating, as well as a collimating optic. The front facets of the gain elements/emitters are coated with an anti-reflection (AR) coating, thus insuring that the external cavity sets the lasing wavelength of the individual gain elements/emitters. Preferably within each IBC cavity is at least one optical element that reduces the divergence of the individual gain elements/emitters in the fast axis while having minimal impact on the divergence in the slow axis.
In one embodiment of the invention, three independent IBC systems are used to generate the three colors (e.g., red, green and blue) necessary for a full color display. The output from each IBC system is modulated and the three beams are combined, for example using dichroic beam splitters, to form a single, modulated output beam. The single output beam is then directed to a scanner that generates the display. In one configuration the scanner is an x-y scanner, thus generating the display by scanning a single output beam across the display. In another configuration, the scanner is a one dimensional scanner, this type of scanner being used when the modulated IBC system creates an entire row or column of the desired display.
In another embodiment of the invention, the three IBC systems required for a full color display utilize a common output coupler. This system can be used to reduce the overall system size and complexity, thus further reducing cost. Although a single collimating optic can be used, preferably each IBC system uses a dedicated collimating optic, thus achieving diffraction limited output with simple spherical optics.
In another embodiment of the invention, each IBC system utilizes a beam combining plate to incoherently combine the outputs of a plurality of emitters (e.g., single, transverse mode edge emitters). The emission of each individual emitter passes through a collection/collimating optic, a prism and a wavelength separating element prior to entering the beam combining plate. In at least one embodiment, the wavelength separating elements (e.g., dielectric coating, holographic grating, etc.) are deposited directly onto an exterior surface of the beam combining plate.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.